1. Field of the Invention
The present invention relates to an apparatus for receiving a T-DMB signal
2. Description of the Related Art
FIG. 1 is a view showing the structure of a transmission frame used in a T-DMB system. The transmission frame 100 comprises a synchronization channel 101, a fast information channel 102 and a main service channel 103. The synchronization channel comprises a null symbol 104 and a phase reference symbol (PRS) 105, and is used for time and frequency synchronization at the receiver side. The fast information channel 102 carries control information for data decoding of the main service channel. The main service channel 103 is a channel containing real video, audio and data information.
FIG. 2 is a block diagram showing the structure of a conventional software-based T-DMB receiver. The receiver 200 comprises an RF tuner 201, an analog-to-digital converter (ADC) 202 and a baseband signal processing unit 203.
The RF tuner 201 amplifies an input signal having a small amplitude and converts a RF frequency into an intermediate or baseband frequency. Further, the RF tuner 201 selects a desired channel through a filter.
The ADC 202 converts analog signals into digital signals.
The baseband signal processing unit 203 comprises an initial synchronizing unit 205 and a data demodulating unit 206.
The initial synchronizing unit 205 comprises a frame synchronizer (205-1), a symbol time synchronizer (205-2) and a frequency synchronizer (205-3). The initial synchronizing unit 205 estimates time and frequency offsets using a null symbol and a PRS and compensates for the estimated time and frequency offsets. The time and frequency offsets estimated through an initial synchronizing process are used for time offset tracking and frequency estimation and compensation together with data demodulation.
The data demodulating unit 206 comprises a time offset tracker 207, an I/Q demodulator (210), a frequency compensator (211), a fractional frequency synchronizer (212), an ODFM demodulator (213), a de-intetleaver (209), a convolution decoder (214), an RS sync byte checker (215) and an RS decoder (216). The data demodulating unit 206 performs I/Q demodulation and OFDM signal modulation, and performs a channel decoding process.
When the receiver 200 is turned on, the initial synchronizing unit 205 is first starts operating. The initial synchronizing unit 205 estimates and compensate for time and frequency offsets using some initial data stored in an input buffer 204. After the initial synchronization is performed, the initial synchronizing unit 205 stops operation, and the data demodulating unit 206 starts operation. If the receiver enters into a tunnel or a shadow area between high-rise buildings, the strength of an input signal received by the receiver is considerably decreased. The RF tuner 201 transmits a received signal strength indication (RSSI) as a level of a received signal to a controller.
If the strength of the input signal is decreased lower than sensitivity, data demodulation is not smoothly performed. However, time and frequency synchronization is generally performed even at the strength of an input signal, where data demodulation is not smoothly performed.
The time offset tracker 207 tracks multiple paths of each frame using a PRS and selects a path along which an input signal is strongest. However, when the receiver enters into a place such as a long tunnel provided with no repeater, the strength of an input signal is extremely decreased. At this time, it is difficult to properly estimate time and frequency offsets any more and loose synchronization. In this case, a controller 208 stops the operation of the data demodulating unit 206 and re-operates the initial synchronizing unit 205 so as to perform time and frequency synchronization.
If the strength of the input signal is maintained lower than reference sensitivity, an initial synchronization process is repeatedly performed until the time and frequency offsets are properly estimated. Therefore, if a receiver receives T-DMB broadcasting while a user moves in a place such as a metropolis having many shadow areas, the aforementioned situation frequently occurs. If the receiver frequently enters into shadow areas, time and frequency synchronization is performed but data demodulation is not properly performed as the strength of an input signal becomes weak. If a data seriously damaged due to the weak strength of input signal is inputted, the damaged data has an influence on subsequent frames due to characteristics of the de-interleaver 209 included in the T-DMB receiver. Although the data demodulating unit 206 is operated for data processing, images or sounds are interrupted as a bad result, and therefore, power is wasted. Further, if the initial synchronizing unit 205 is re-operated as the strength of the input signal becomes weaker, the initial synchronization process is continuously repeated, and therefore, power is also wasted. Furthermore, a large amount of calculation is necessary for the channel decoding process and the operation of the initial synchronizing unit 205. For this reason, if the large amount of calculation is not controlled, power consumption is serious.